Generally, a wire cut electric discharge machine comprises upper and lower nozzles respectively provided at upper and lower wire guides, and machining fluid supply pipe lines connected to these nozzles, and is arranged to inject machining liquid from the upper and lower nozzles toward an electric discharge machining region between a workpiece and a wire electrode, to thereby remove the sludge and heat produced during electric discharge machining. For rough machining, the opening degrees of flow regulating valves respectively provided in the machining fluid supply pipe lines are increased, to thereby supply the machining fluid in an amount large enough to remove the sludge and heat that are produced in large quantities during the rough machining. On the other hand, in a finishing operation including a second cut, the valve openings are decreased, so as to reduce a deviation of the wire electrode due to the injection pressure of the machining fluid, to thereby improve machining accuracy.
In the aforementioned type of electric discharge machine, it is known to carry out automatic wire extension by using the machining fluid. Namely, at the time of wire extension, the machining fluid is jetted toward the workpiece from a jet nozzle mounted to the upper nozzle, while the injection of the machining fluid from the lower nozzle is interrupted. Then, the wire electrode is fed from the upper wire guide, while being restricted by the jet of the machining fluid, so that the wire passes through a machining initial hole formed in the workpiece and the wire passage of the lower wire guide, thereby effecting the wire extension.
During a finishing operation, however, since the openings of the flow regulating valves in the machining fluid supply pipe lines are small, the amount of the machining fluid supplied to the jet nozzle is reduced, and hence the injection pressure of the jet stream of machining fluid is lowered. The thus reduced injection pressure sometimes causes an unsuccessful wire extension performed in the middle of a finishing operation.